For many communication devices, microwave filters are an integral block. Filters having high performance while utilizing small silicon area are highly sought after because commercial market trends are constantly striving towards low cost and highly integrated solutions.
Silicon based manufacturing processes have been the dominant leader in low cost and ability to achieve a high level of integration. However, passive filters have difficulty being integrated onto silicon substrates. Firstly, passive devices are silicon area hungry and lump elements or transmission line filters are known to occupy huge areas, often making them too impractical to be integrated on an integrated circuit chip. This situation is made worse in lower frequency applications where the required sizes for effective operation of such low frequency filters are even larger. Secondly, silicon substrates are very lossy which has a huge impact on integrated filters. The lossiness of the silicon substrates results in poor filter performance. And at higher frequencies, substrate losses are more pronounced.
Consequently, in a bid to achieve a high level of integration, typical filter development is based upon active filter topologies. Active filters, however, consume current, making them not very suitable for wireless mobile applications which operate from a limited battery source.
There have been numerous works and publications on the fabrication of passive filters based on silicon process. For example, a planar ring resonator has been fabricated within a CMOS manufacturing process. The ring resonator includes a perturb stub and functions as a dual mode bandpass filter at 60 GHz. Yet the silicon area required for fabrication is still large and the resonator has poor selectivity. In another implementation, a bandpass filter operating at 77 GHz has been fabricated within a Silicon Germanium (SiGe) manufacturing process. The filter employs lumped elements, spiral inductors and metal insulator metal (MIM) capacitors, yet still suffers from high insertion loss. Other attempts include attempts to incorporate a filter monolithically.
In a bid to achieve high integration, film bulk acoustic resonators (FBAR) are used in conventional filter designs. Unlike ceramic and surface acoustic filters (SAW), fabrication of FBAR filters is compatible with conventional integrated circuit materials and manufacturing technologies. FBAR devices include a piezoelectric layer, such as zinc oxide or aluminum nitride, sandwiched between two electrodes and positioned above a cavity in a substrate. Through a combination of series and shunt FBAR resonators in a ladder configuration, bandpass filtering can be performed. Similar to FBARs, high aspect ratio interlocking transducers (IDTs) and reflectors allow CMOS based SAW resonators to achieve GHz frequencies with high quality functionality. However, FBAR or SAW filters implemented with current technologies are limited to those operable at frequencies below 5 GHz.
Through the use of thru-silicon-via (TSV) structures, another bandpass filter design can be realized. Parallel TSV structures can form a pair of electrically coupled lines to perform filtering functions. However, FBAR, SAW and TSV structures all require special processing steps, limiting their scalability into standard IC technologies such as CMOS, SiGe BiCMOS, GaAs and InP. These special processing steps are not within standard silicon fabrication processes, thereby requiring special care and materials which leads to both cost and reliability issues.
Thus, what is needed is a high performance filter design requiring small silicon area while being highly scalable into one or more of the conventional integrated circuit fabrication technologies. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure.